Technical Field
The present disclosure relates to a testing head comprising a plurality of vertical probes having an improved sliding movement within respective guide holes and correct holding within the head itself.
More specifically, the disclosure refers to a testing head comprising vertical probes for the functionality testing of an electronic device, the testing head comprising at least one couple of plate-like supports separated from each other by a suitable gap and provided with respective guide holes in order to slidably house a plurality of contact probes, each contact probe comprising a rod-like body extending according a preset longitudinal axis between a first and a second end, the first end being a contact tip adapted to abut onto a contact pad of a device under test, and the second end being a contact head adapted to abut onto a contact pad of a space transformer.
The disclosure relates in particular, but not exclusively, to a testing head for testing electronic devices integrated on semiconductor wafers and the following description is made with reference to this field of application with the only purpose of simplifying its exposition.
Description of the Related Art
As it is well known, a testing head (or probe head) is a device adapted to electrically connect a plurality of contact pads of a microstructure with corresponding channels of a testing machine that performs the functionality testing thereof, in particular the electrical one, or generically the test.
The test, which is performed on integrated circuits, is particularly useful to detect and isolate defective circuits already in the manufacturing step. Usually the testing heads are thus used to electrically test the circuits that are integrated on a wafer before cutting and assembling them inside a chip containment package.
A testing head generally comprises a plurality of movable contact elements or contact probes held by at least one couple of supports or guides, which are substantially plate-shaped and parallel to each other. Such plate-shaped supports are provided with suitable holes and arranged at a certain distance from each other in order to leave a free zone or gap for the movement and the possible deformation of the contact probes. The couple of plate-shaped supports includes in particular an upper plate-shaped support and a lower plate-shaped support, both provided with guide holes within which the contact probes axially slide, the probes being usually made of wires of special alloys having good electrical and mechanical properties.
The good connection between the contact probes and the contact pads of the device under test is ensured by pressing the testing head on the device itself, the contact probes, which are movable within the guide holes made in the upper and lower plate-shaped supports, undergoing a bending inside the gap between the two plate-shaped supports and a sliding within such guide holes during that pressing contact. Testing heads of this type, i.e. testing head with vertical probes, are usually called “vertical probe heads”.
The testing heads with vertical probes typically have a gap within which a bending of the contact probes occurs, and such bending can be assisted by a suitable configuration of the probes themselves or of their supports, as schematically shown in FIG. 1, where for sake of illustration simplicity only one contact probe of the plurality of probes usually included in a testing head has been shown.
In particular, FIG. 1 schematically illustrates a testing head 1 comprising at least one lower plate-shaped support 2, usually indicated as “lower die”, and one upper plate-shaped support 3, usually indicated as “upper die”, having respective guide holes 4 and 5 within which at least one contact probe 6 slides.
The contact probe 6 terminates at one end with a contact tip 7 intended to abut onto a contact pad 8 of a device under test 10, so as to achieve the mechanical and electrical contact between said device under test 10 and a test apparatus (not shown) of which such testing head forms a terminal element.
Here and hereinafter the term “contact tip” means an end zone or region of a contact probe designed to contact the device under test or the testing apparatus, said contact zone or region being not necessarily sharpened.
In some cases, the contact probes are fixedly fastened to the head itself in correspondence of the upper plate-shaped support: such testing heads are referred to as “blocked probe testing heads”.
More frequently, however, testing heads with unblocked probes, i.e. probes not fixedly fastened, are used, those probes being interfaced to a so-called board, possibly through a micro-contact board: such testing heads are referred to as non-blocked probe testing heads. The micro-contact board is usually called “space transformer” as, besides contacting the probes, it also allows to spatially redistribute the contact pads made on it with respect to the contact pads on the device under test, in particular relaxing the distance constraints between the centers of the pads themselves.
In this case, as illustrated in FIG. 1, the contact probe 6 has a further contact tip 15 towards a pad 11 of a plurality of contact pads of the space transformer 12. The good electrical contact between probes 6 and space transformer 12 is guaranteed in a similar manner to the contact with the device under test 10 by pressing the contact tips 15 of the contact probes 6 on the contact pads 11 of the space transformer 12.
The lower and upper plate-shaped supports 2 and 3 are suitably spaced by a gap 9 that allows the deformation of the contact probes 6. Finally, the guide holes 4 and 5 are sized so as to allow a sliding of the contact probe 6 therein.
The shape of the deformation undergone by the probes and the force required to produce such deformation are dependent on numerous factors, such as the physical characteristics of the alloy which makes the probes and the value of the offset between the guide holes in the upper plate-shaped support and the corresponding guide holes in the lower plate-shaped support.
The correct operation of a testing head is basically linked to two parameters: the vertical movement, or overtravel, of the contact probes and the horizontal movement, or scrub, of the contact tips of such contact probes. As is known, it is important to ensure the scrub of the contact tips so as to allow to scrub the surface of the contact pads, in this way removing the impurities that are for example in the form of a thin layer or oxide films, thus improving the contact carried out by the testing head.
All these features are evaluated and calibrated in the manufacturing step of a testing head in such a way that the good electrical connection between the probes and the device under test is always guaranteed. In the case of a testing head realized with the so-called “shifted plates” technology, the contact probes 6, which are also referred to as “buckling beam”, are made straight, with a constant cross-section throughout their length, preferably rectangular, and usually they are sharpened at their ends in order to form the contact ends, in particular the contact tip 7 and the contact head 15 respectively, as shown in FIG. 1. It is known to manufacture this type of testing heads by superimposing the plate-shaped supports in order to place the respective guide holes in a mutual correspondence, to fit the contact probes in those guide holes, to space the plate-shaped supports to form the so-called gap and then to offset or to shift such supports, causing a deformation of the probe body, in a substantially central position, as shown in FIG. 1. These testing heads are also referred to as “shifted plate testing heads”.
It is also well known that such testing heads with probes of the “buckling beam” type have considerable problems of friction of the contact probes within the respective guide holes, which can make it difficult or even prevent the sliding of the probes therein.
In extreme cases, such a friction can thus cause the probes to get stuck in the relative guide holes, in particular in correspondence of the contact points between the sides of the probe and the walls of the guide holes of the plate-shaped supports, for example during the vertical displacement or overtravel of the probe for the pressing contact of the contact tip on a contact pad of a device under test. Then undesirably, during the testing operations, a non optimal contact between the probe and the contact pad is obtained, with consequent efficiency problems of the electrical test carried out by the testing head as a whole.
The decrease of such sliding friction of the probes is in some cases achieved thanks to the use of a plurality of plate-shaped guides, preferably two guides, which are parallel to each other in order to form at least one of the plate-shaped supports of the head.
In such case, however, mechanisms are usually provided in order to prevent the probes from undesirably escaping from the testing heads, in particular during the absence of the device under test onto which the probes abut during the normal operation of the testing head.
A testing head comprising non-blocked probes comprising a coating having a transversal dimension greater than that of the guide holes of the lower plate-shaped support, so as to avoid the fall by gravity of the contact probes, is described for example in U.S. Pat. No. 6,150,830 to Schmid et al. Other holding systems of the probes are described in the German patent application No. DE-A-3337915 in the name of Feinmetall GmbH and in US patent application published with No. US 2014/0118016 to Breinlinger et al., as well as in European patent application No. EP 2110673 in the name of the Applicant itself.
Although advantageous under several aspects, the known solutions have some drawbacks; in particular, some of the testing heads made according to the known art are not yet able to prevent the probes from escaping from the guide holes, while other solutions introduce problems due to the probes getting stuck inside the guide holes, in particular during the movement in a direction orthogonal to the device under test of the respective contact tips. In yet other cases, the overall dimension of the probes is increased in an unacceptable way.
The technical problem of the present disclosure is thus to provide a testing head for microstructures having functional and structural characteristics such to allow the minimization of the friction forces existing between the lateral walls of the contact probes and the walls of the corresponding guide holes of the upper and lower supports, while ensuring a correct holding of the probes inside the testing head, so as to overcome the limits and drawbacks currently affecting the testing heads for microstructures made of the known art.